1. Field of the Invention
The present invention relates to a printing apparatus such as an ink-jet plotter having a linear encoder which detects print head position and controls printing position.
2. Description of the Related Art
A printing apparatus like an ink-jet plotter having a movable print head comprises a linear encoder which detects position of the print head. In such printing apparatus, the print head is driven along a guide shaft. A position information signal indicating position of such movable print head is generated based on output signals of the linear encoder. The print head is driven based on the position information signal and image data. The linear encoder comprises a scale (timing fence) and a sensor. The scale is disposed so as to be in parallel to the guide shaft. The sensor is coupled with the print head, and measures graduation on the scale while being driven along the scale.
The linear encoder outputs an A-phase output signal and a B-phase output signal as shown in FIG. 10. The phase difference between the A-phase output and the B-phase output is 90 degrees. A position information signal for determining pitch of printing position is generated based on the A-phase and B-phase outputs from the linear encoder. In a case where a plotter is designed for printing images on a sheet in relatively large size such as A1 or A0, the plotter requires a longer scale. However, it is difficult to obtain high resolution from such a long scale. In order to obtain higher printing resolution than the actual scale resolution, a position information signal with high resolution has to be formed from the A-phase or B-phase outputs. For example, as shown in FIG. 10, a position information signal whose frequency is four times higher than that of the A-phase and B-phase outputs may be formed by detecting the leading and trailing edges of the A-phase and B-phase outputs. With the position information signal, the resultant resolution for printing will be 720 dpi (dots per inch) while the scale resolution is 180 dpi.
Such a conventional method of controlling printing position in an ink-jet plotter, however, has problems of that (i) increased pulses in the print position information signal may be inconstant when encoder's error occurs as shown in FIG. 11, and this causes low image quality; and (ii) the selectable resolution is limited to few resolution sets, and the maximum resolution is only four times higher than the scale resolution.
In consideration of the above, the inventor of the present invention has proposed an improved printing apparatus, in Laid-open Japanese Patent Application (kokai) No. 9-254480, which provides arbitrary selectable printing resolutions while controlling the print head with high resolution. In the printing apparatus, it measures intervals between edges of the outputs from the encoder, divide the measured interval data by a predetermined number, and generate pulses in each divided time periods. Thus, the higher resolution is realized with the generated pulses for printing position control.